1. Field of the Invention
The present invention relates to a band-pass filter and method using a transmission line therefor, and more particularly to a compact band-pass filter which can be completely integrated and implemented using a multilayer interdigital capacitor as a capacitor compensation circuit.
2. Description of the Related Art
With the spread of wireless mobile communications, existing frequency resources are becoming saturated, resulting in higher and higher frequency bands being additionally used. This has caused an increase in demand for band-pass filters which can be used in higher-frequency bands. In the field of wireless communication equipment, such as cellular phones or a wireless LANs, size and cost are of great concern, and thus diverse techniques for reducing the size and cost have been developed.
Specifically, in order to reduce the size of a product, a structure using transmission lines that can be implemented in small spaces has been used instead of a lumped passive element which would occupy a large space. Its representative example would be a band-pass filter implemented by transmission lines, used for the purpose of extracting only a signal of a desired frequency band and intercepting other noise signals. This band-pass filter has been used in diverse fields, including in wireless communication systems. In a wireless communication system, the band-pass filter has been used to receive or transmit only a desired signal in a transmitter or a receiver.
A filter implemented by use of a stripline, which is a type of transmission line, is disclosed in U.S. Pat. No. 4,963,843, issued to Motorola, Inc. on Oct. 16,1990. With reference to those described in U.S. Pat. No. 4,963,843, a conventionally proposed combline stripline filter will be explained.
The conventional combline stripline filter includes conductive strips. One end of the conductive strip is connected to ground, and the other end thereof is loaded to ground so as to have a capacitance. That is, in a substrate having an uppermost surface and a lowermost surface that constitute the combline stripline filter, the uppermost surface and the lowermost surface are ground surfaces. Meanwhile, an inner circuitry layer is formed between the uppermost surface and the lowermost surface. A ground area has angled edges formed by a predetermined number of substrate surfaces crossing one another, and is coupled to ground planes. One end of the combline resonator, which corresponds to the inner circuitry layer, is coupled to the ground planes, and the other end thereof is coupled to the ground area so as to have a capacitance. However, the above-described combline stripline filter includes drawbacks of having a complicated structure and large size, while further requiring input/output ports being provided through via-holes.
Another example of a filter using the above-described transmission lines and a combline structure is disclosed in U.S. Pat. No. 6,762,659. This patent proposes a wireless filter of a combline structure which has a capacitor compensation circuit that connects respective layers constituting a multilayer structure through via-holes. In this case, the wireless filter of the combline structure has a capacitor compensation circuit, provided with a capacitor of a lumped element, as a capacitance compensation unit.
In the above-described structure, a parasitic effect generated in the connection part of the lumped element and the transmission line increases as the frequency band becomes heightened, and therefore, it may be difficult to implement. Since the lumped element should be attached to the transmission line by surface mounting, an additional packaging cost is also incurred, as well as its manufacturing process being complicated.
Although filters implemented by use of transmission lines have been used in many wireless mobile communication modules, due to its good integration, their size is dependent on the frequency band. In particular, in a C-band, which is used as the frequency band for Bluetooth devices or wireless LANs, the size of the filter is relatively large, and thus a scheme for reducing the size is required.